According to Au7o's research across NHTSA recalls, manufacturer TSBs, and owner forum reports, the 2004 Toyota Highlander has 4 documented known issues, with 3 rated critical. The most serious are 2.4L 2AZ-FE Head Gasket and Head Bolt Thread Failure ($2,500-$5,000 repair), Radiator Internal Transmission-Cooler Failure — ATF/Coolant Cross-Contamination ("Strawberry Milkshake") ($400-$4,500 repair) and V6 Engine Oil Sludge / Gelling (1MZ-FE / 3MZ-FE) — Oil Pump Pickup Starvation ($500-$6,000 repair). Across all issues with published estimates, repair costs range from $200 to $6,000. DIY maintenance guides at au7o.io.